ething of considerable interest. Look out to sea!"

Dick turned his head involuntarily. He heard Von Kettler's laugh,
heard the snap of a switch--then suddenly he was alone in the void.

At that snap of the switch, everything had vanished from view behind
him, the building, even the platform on which he stood. His feet
seemed to rest on nothing. Yet below him he could still see the
airplanes, and more being wheeled out.

* * * * *

A sense of extreme physical nausea overcame him. He reeled, then
managed to steady himself. He, too, was invisible to his own eyes.
Involuntarily he cried out. No sound came from his lips. He stood
there, invisible in an invisible, soundless void.

For what seemed an unending period he occupied himself with
endeavoring to obtain the sense of balance. Then, with a great effort,
he managed to loosen the cords that bound his right arm to his side. A
mighty wrench, and he had slipped them up above his elbow. His right
lower arm was free.

He extended it cautiously, and his hand encountered a railing.
Instantly he felt more at ease. He began moving slowly around in a
widening circle, and discovered that the platform was enclosed. The
further side was, however, open, and he began sliding forward, foot by
foot, to locate himself. Once his foot slipped over the edge, and he
drew back hastily. He felt on the other side, and discovered that he
was upon what seemed a plank walk, perhaps a hundred and fifty feet
above the ground, with no rail on either side, and some six feet wide.

Very cautiously he shuffled his way along it. It was solid enough,
although invisible, but more than once Dick walked perilously close to
one edge or the other. At length he went down on his hands and knees,
and proceeded, crawling, until his movements were arrested by what was
unmistakably a door.

The plank bridge, then, connected the top stories of two buildings,
but what the second was, there was no means of knowing. The door was
barred on the other side, and did not yield an iota to Dick's cautious
pressure. Dick felt the frame. Beyond was glass, reinforced with iron
on the outside, the latter metal forming a sort of lattice work.
Cautiously Dick began to crawl up the rounded dome.

* * * * *

Foot by foot he made his way, clinging to the iron bars, until he felt
that he had reached the point of the dome's maximum convexity. He
wedged his feet agai